Routine voluntary HIV testing with the right to decline (the "opt-out" approach) is recommended for all pregnant women in the United States but not all are tested. We examined data from surveys of prenatal care providers to identify factors associated with universal testing among patients. Data are from a probability survey conducted in 1999 with prenatal care providers in four separate areas. Survey participants completed a self-administered questionnaire. We computed the percentage of providers reporting universal testing by characteristics of the provider's practice, medical specialty, how strongly they encouraged testing, perceptions of patients' risk, and whether they saw patients in public clinics. In the four locations (Brooklyn, New York; North Carolina; Connecticut; and Dade County, Florida) 95%-99% of providers reported that they routinely offered HIV testing to all pregnant women; the average percentage tested was 64%-89%. The percentage reporting that all of their patients were tested ranged from 12%-62%. The percent of providers reporting universal testing was positively associated with the degree to which testing was encouraged, particularly encouragement to women perceived to be low risk. In some areas, universal testing varied by medical profession, with obstetric physicians and residents, and nurse-midwives reporting a lower percentage of universal testing than family practice physicians and residents. To achieve the goal of routine HIV testing of all pregnant women, education and training must be delivered to all prenatal care providers. This training should emphasize the importance of routine testing. Even with the opt-out approach, many women may decline testing if their doctor does not recommend and encourage HIV testing.